Interconnection of lead frame to die utilizing flip chip process

ABSTRACT

Embodiments in accordance with the present invention relate to techniques which avoid the problems of deformation in the shape of a solder connection in a flip chip package, resulting from solder reflow. In one embodiment, a solder-repellent surface is created adjacent to the solder to constrain the reflow and thereby maintain the vertical profile of the solder. Examples of such a solder-repellent surface include an oxide (such as Brown Oxide) of the lead frame, or a tape (such as Kapton) which is used as a dam bar to control/constrain the solder flow on the leads prior to the encapsulation step. In another embodiment, the solder connection may be formed from at least two components. The first component may reflow at high temperatures to provide the necessary adhesion between solder ball and the die, with the second component reflowing at a lower temperature to provide the necessary adhesion between the solder ball and the leads. An example of such multi-component connections include a first high temperature reflow solder ball paired with a second low temperature reflow solder. Another example includes a solder ball with a hard core (such as Cu, stainless steel, or a plastic material stable at high temperatures) coated with a lower temperature reflow material.

CROSS-REFERENCE TO RELATED APPLICATION

The instant nonprovisional patent application claims priority to U.S.Provisional Patent Application No. 61/126,243, filed May 1, 2008 andincorporated by reference herein for all purposes.

BACKGROUND OF THE INVENTION

FIG. 1 shows a simplified perspective view of a conventional flip chippackage 100. FIG. 1A shows a cross-sectional view of the package of FIG.1 taken along line 1A-1A′.

This conventional flip chip package 100 comprises die 102 enclosedwithin plastic package body 104. Electrical contacts 106 on the topsurface of die 102 are in electrical communication with the leads 108projecting from the package body, through electrically conducting solderconnections 110.

As indicated in FIG. 1A, it is required that the leads 108 maintain aminimum height Z over the die. However, as shown FIG. 1B, during thestep of attaching the leads to the die, heating of the solder connectionmay result in solder reflow, causing the shape of the solder connectionto deform and possibly resulting in the height of the lead Z′ being lessthan the required height Z.

Accordingly, there is a need in the art for improved techniques forfabricating a flip chip die which avoids the problems resulting fromdeformation in a solder contact resulting from reflow.

BRIEF SUMMARY OF THE INVENTION

Embodiments in accordance with the present invention relate totechniques which avoid the problems of deformation in the shape of asolder connection in a flip chip package, resulting from solder reflow.In one embodiment, a solder-repellent surface is created adjacent to thesolder to constrain the reflow and thereby maintain the vertical profileof the solder. Examples of such a solder-repellent surface include anoxide (such as Brown Oxide) of the lead frame, or a tape (such asKapton) which is used as a dam bar to control/constrain the solder flowon the leads prior to the encapsulation step. In another embodiment, thesolder connection may be formed from at least two components. The firstcomponent may reflow at high temperatures to provide the necessaryadhesion between solder ball and the die, with the second componentreflowing at a lower temperature to provide the necessary adhesionbetween the solder ball and the leads. An example of suchmulti-component connections include a first high temperature reflowsolder ball paired with a second low temperature reflow solder. Anotherexample includes a solder ball with a hard core (such as Cu, stainlesssteel, or a plastic material that is stable at high temperatures) thatis coated with a lower temperature reflow material.

These and other embodiments of the present invention, as well as itsfeatures and some potential advantages are described in more detail inconjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified perspective view of a conventional flip chippackage.

FIGS. 1A-B shows simplified perspective views a conventional flip chippackage.

FIG. 2 shows a simplified shows a simplified cross-sectional view of oneembodiment of the present invention, which utilizes a non-wettable oxidesurface on a lead finger, to constrain solder reflow.

FIG. 3A shows a plan view of an embodiment of the present invention,which utilizes an adhesive, non-wettable tape material that is stable athigh temperatures, on a lead finger, to constrain solder reflow.

FIG. 3B shows a simplified shows a simplified cross-sectional view ofthe embodiment of the present invention shown in FIG. 3A.

FIG. 3C shows a plan view of an alternative embodiment of the presentinvention utilizing tape material to constrain solder reflow.

FIG. 4 shows a simplified shows a simplified cross-sectional view of oneembodiment of the present invention, which utilizes a solder contactcomprising a second material having a reflow temperature lower than afirst material.

FIG. 5 shows a simplified shows a simplified cross-sectional view of oneembodiment of the present invention, which utilizes a solder contactcomprising a hard core material having a reflow temperature that ishigher than that of an outside coating.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments in accordance with the present invention relate totechniques, employed alone or in combination, which avoid the problemsof deformation in the shape of a solder connection in a flip chippackage, resulting from solder reflow.

In certain embodiments, a solder-repellent surface is formed on thesurface of the lead adjacent to the surface expected to be in contactwith the solder connection. This solder-repellent surface constrainsreflow of the solder and thereby maintains the requisite verticalspacing between the lead and the die.

FIG. 2 shows a simplified cross-sectional view of one embodiment of thepresent invention. In FIG. 2, lead 208 is provided having portion 208 aof its surface oxidized (for example Brown Oxide), and other portions208 b substantially free of oxide. Such oxidation of only portions ofthe lead may be accomplished by masking selected portions, followed byexposure of unmasked portions to an oxidizing ambient.

Solder ball 210 is provided attached to pad 206 present on the topsurface of the die. As particularly shown in FIG. 2, the solder ball issupported on an electroless nickel immersion gold (ENIG) material 220,that is present within an opening of passivation 222 covering the topsurface of the die. Alternatively or in conjunction with ENIG, thesolder ball may be supported on a wettable surface provided bysputtering.

In the step of attaching the lead 208 to the die via the solder contact,the solder ball is heated to above its reflow temperature. As shown inFIG. 2, however, the presence of the oxidized lead portions 208 aadjacent to the oxide-free lead portion 208 a in contact with thesolder, serve to restrain the flow of heated solder. In particular, thenon-wettable character of the oxide inhibits the spread of the moltensolder. Thus, while the portions of the solder distal from the lead maybulge, the solder proximate to the lead is constrained from flowing, andthereby allows the solder to substantially maintain its verticalprofile.

On the other side of the solder contact, the combination of the ENIGmaterial 220 within the opening in the passivation, similarly serves toconstrain the flow of molten solder, thereby preserving the verticalprofile.

It is noted that obtaining a completely oxide-free surface to receivethe solder may be difficult to achieve, given the prevalence of oxygenin the environment. Accordingly, prior to bringing the lead into contactwith the molten solder, the solder may be dipped into a flux materialthat serves to remove any native oxide.

While the particular embodiment just described utilizes a bare Cusurface to receive the solder, this is not required by the presentinvention. In accordance with alternative embodiments, the surface thatis configured to receive the solder may comprise a plated metal such assilver, or a stack of plated stack of metal such as Ni/Au or Ni/Pd/Au.In such embodiments, the wettability of the plated metal/metal stack mayallow only a native oxide layer (rather than an intentionally grownBrown Oxide layer) to contain the flow of solder in the desired manner.

And while the particular embodiment just described utilizes asolder-repellent, non-wettable surface in the form of an oxide, this isnot required by the present invention. In accordance with alternativeembodiments, other materials could be employed to limit the spread ofsolder and resulting deformation in the shape of the solder contact. Forexample in alternative embodiments portions of the lead could be coatedwith a non-wettable polymer material configured to withstandtemperatures in excess of the solder reflow temperature.

In yet another embodiment, the flow of molten solder could beconstrained by the presence of a tape. For example, during fabricationof a flip chip package, it may be necessary to maintain the orientationand spacing of a number of leads to allow those leads to match with thecorresponding locations of contacts on the top of the die. In order toensure this lead orientation and spacing is maintained, the leads may besecured to an adhesive tape such as Kapton. This is shown in theunderside view of FIG. 3A, wherein the bottoms of a plurality of leads308 are maintained in place by tape 330.

In accordance with an embodiment of the present invention, this tape 330may be utilized to constrain the reflow of solder. FIG. 3B shows asimplified cross-sectional view, wherein tape 330 is present proximateto the edge of lead 308, leaving an end portion 308 a of the lead 308uncovered.

Solder ball 310 is provided attached to pad 306 present on the topsurface of the die. As particularly shown in FIG. 3, the solder ball issupported on ENIG material 320 present within an opening of passivation322 covering the top surface of the die. Alternatively or in conjunctionwith ENIG, the solder ball may be supported on a wettable surfaceprovided by sputtering.

In the step of attaching the lead 308 to the die via the solder contact,the solder ball is heated to above its reflow temperature. As shown inFIG. 3B, however, the presence of tape 330 adjacent to lead portion 308a in contact with the solder, restrains the flow of heated solder. Inparticular, the non-wettable character of the tape inhibits the spreadof the molten solder. In addition, surfaces of the lead such as thevertical edge of the lead, may also bear a non-wettable oxide thatfurther inhibits the flowing of the molten solder.

Thus as shown in FIG. 3B, while the portions of the solder distal fromthe lead may bulge, the solder proximate to the lead is constrained fromflowing, and thereby allows the solder to substantially maintain itsvertical profile.

While the above embodiment illustrates an arrangement of a tape incontinuous rectangle configuration, this is not required by the presentinvention. In accordance with alternative embodiments, strips of tapecould be used as shown in FIG. 3C. Further alternatively, tape in theform of adjacent pairs of strips or pairs of rectangles could beutilized to constrain the reflow of solder between them.

In accordance with certain embodiments, the thickness of the tape mayact a physical impediment to the flow of solder. Moreover, in accordancewith certain embodiments, use of tape of a predetermined thickness mayserve to maintain the desired spacing between the lead and the die. Insuch an embodiment, the tape serves not only as a non-wettable surface,but its bulk physically supports the lead over the die.

Moreover, while the above embodiment is described in conjunction withthe use of a Kapton tape, this is also not required by the presentinvention. A variety of tape materials exhibiting stability at hightemperatures and the desired adhesive and non-wettable property, couldbe employed to constrain molten solder flow. Any organic material thatcan withstand the high reflow temperature, with pressure sensitiveadhesive on one side of the film to adhere/attach to the leads, can beused.

While the above embodiment illustrates an arrangement of a tape inrectangle, this is not required by the present invention. In accordancewith alternative embodiments, strips of tape could be used as shown inFIG. 3C.

In accordance with alternative embodiments, a vertical profile of asolder connection may be maintained despite reflow, utilizing a solderconnection comprising at least two components. Specifically, a firstcomponent of the solder may reflow at a higher temperatures to providethe necessary adhesion to the die, while remaining as a solid at a lowerreflow temperature to provide the desired vertical spacing. The secondcomponent of the solder may reflow at the second, lower temperature, toprovide adhesion between the first solder component and the lead finger.

FIG. 4 shows a simplified cross-sectional view of one example of such anembodiment in accordance with the present invention. In particular,solder ball 410 is provided attached to pad 406 present on the topsurface of the die 402. As particularly shown in FIG. 4, the solder ballis supported on ENIG material 420, that is present within an opening ofpassivation 422 covering the top surface of the die. Alternatively or inconjunction with ENIG, the solder ball may be supported on a wettablesurface provided by sputtering.

FIG. 4 shows that lead 408 is provided with an electrically conductingadhesive material 440 exhibiting a lower reflow temperature than thematerial of the solder ball 410. Upon heating, material 440 reflows,allowing adhesive attachment with the solder ball 410. The materialcomprising solder ball 410, however, remains solid at this temperature,and its bulk ensures that the desired height Z between the lead and thedie, is maintained.

The following TABLE provides a listing of electrically conductingadhesive materials which can be used according to embodiments of thepresent invention together with their respective reflow temperatures:

TABLE REFLOW MATERIAL COMPOSITION (%) TEMPERATURE (° C.) Solder SAC 101Sn 98.9, Ag 1.0, Cu 0.1 217 Solder SAC 105 Sn 98.5, Ag 1.0, Cu 0.5 217Solder SAC 405 Sn 95.5, Ag 4.0, Cu 0.5 217 Sn 96.5/Ag 3.5 221 lead-basedsolder Sn 10/Pb 90, 275 Sn 62/Pb 36/Ag 2, 179 Sn 63/Pb 37 183

During operation of the die, a certain amount of heat would be expectedto be generated. Accordingly, a requirement of the solder materials usedin a package according to an embodiment of the present invention, isthat they remain solid at such expected operating temperatures.

The solder contact serves not only to allow for electrical communicationbetween the die and the lead finger, but also desirably serves as aconduit for thermal energy generated by the die, to be transportedoutside the package through the lead finger for dissipation in theenvironment. Accordingly, the various materials comprising the soldercontact should exhibit a thermal conductivity necessary to allow therequisite flow of heat from the die.

Another example of an approach in accordance with an embodiment of thepresent invention involves the use of a solder contact comprising asolder ball having a hard core made from a first material having arelatively high reflow temperature, and having a coating made from amaterial having a lower reflow temperature.

FIG. 5 illustrates a simplified cross-sectional view of such anembodiment. In particular, solder contact 510 comprising an outermaterial 510 a (coating) and an inner material (hard core) 510 b, isprovided attached to pad 506 present on the top surface of the die 502.The solder contact is provided with the die 502, supported on ENIGmaterial 520, that is present within an opening of passivation 522covering the top surface of the die. Alternatively or in conjunctionwith ENIG, the solder ball may be supported on a wettable surfaceprovided by sputtering.

FIG. 5 shows that upon heating of the solder contact to a reflowtemperature of the outer coating material 510 a, this material reflows,allowing adhesion to the lead finger 508. The core material 510 bcomprising the core of the solder contact, remains solid at thistemperature. In an example, the core of the solder contact may comprisea solder ball, and the coating may comprise an electrically conductingmaterial that reflows at a temperature lower than the melting point ofthe solder ball. The presence of this solid core maintains the spacing Zbetween the lead and the die. Examples of electrically and thermallyconducting materials exhibiting relatively high reflow temperatureswhich could be used as the core material for a solder contact, includebut are not limited to copper, stainless steel, and plastic materialsexhibiting a sufficiently high reflow temperature.

While not specifically mentioned above, in certain embodiments more thanone of the techniques specifically described may be employed incombination. For example, in accordance with particular embodiments thevertical profile offered by a solder contact may be substantiallymaintained above a reflow temperature utilizing both a solder-repellentsurface (such as oxide or tape), and a solder contact structure having afirst component exhibiting lower reflow temperature.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

1. A method of fabricating a package for a semiconductor device, themethod comprising: providing a die having a pad and a solder ball incontact with the pad; providing lead frame comprising a lead fingerhaving a solder repellent surface adjacent to a portion of the leadfinger that is expected to be in contact with the solder ball; andheating the solder ball to above a reflow temperature, such that thesolder ball adheres to the lead finger portion, while the solderrepellent surface constrains reflow of the solder to maintain a verticalprofile of the solder ball and ensure a minimum spacing between the leadfinger and the die.
 2. The method of claim 1 further comprising formingthe solder repellent surface by oxidizing the lead frame.
 3. The methodof claim 2 wherein the oxidizing comprises intentionally forming BrownOxide.
 4. The method of claim 3 wherein the portion of the lead fingercomprises bare Cu, a plated metal, or a plated metal stack.
 5. Themethod of claim 2 wherein the oxidizing comprises allowing a nativeoxide to form.
 6. The method of claim 5 wherein the portion of the leadfinger comprises a plated metal or a plated metal stack.
 7. The methodof claim 6 wherein the plated metal or plated metal stack comprises Ag,Ni/Au, or Ni/Pd/Au.
 8. The method of claim 2 further comprising:pattering a mask on the portion of the lead finger prior to oxidation ofthe lead frame; and removing the mask.
 9. The method of claim 1 furthercomprising forming the solder repellent surface by placing a tape on thelead finger.
 10. The method of claim 9 wherein the tape is comprised ofKapton.
 11. The method of claim 9 wherein the tape is applied tostabilize a plurality of lead fingers prior to an encapsulation process.12. The method of claim 9 wherein a thickness of the tape maintains thelead finger at the minimum spacing between the lead finger and the die.13. The method of claim 1 wherein the die is provided with the solderball in contact with the pad through an opening in a passivation layerand an electroless nickel immersion gold (ENIG) layer that serve toconstrain a flow of solder on a surface of the die.
 14. The method ofclaim 1 wherein the die is provided with the solder ball in contact withthe pad through a sputtered wettable layer that serves to constrain aflow of solder on a surface of the die.
 15. A method of fabricating apackage for a semiconductor device, the method comprising: providing adie having a pad and a solder contact in contact with the pad, thesolder contact comprising, a first component in contact with the pad,the first component having a first reflow temperature, and a secondcomponent having a second reflow temperature lower than the first reflowtemperature; providing a lead frame comprising a lead finger; andheating the solder contact to above the second reflow temperature andbelow the first reflow temperature, such that the first componentensures a minimum spacing between the lead finger and the die, and thesecond component ensures adhesion between the solder contact and thelead finger.
 16. A method of fabricating a package for a semiconductordevice, the method comprising: providing a die having a pad and a soldercontact in contact with the pad, the solder contact comprising, a firstcomponent in contact with the pad, the first component having a firstreflow temperature, and a second component having a second reflowtemperature lower than the first reflow temperature; providing a leadframe comprising a lead finger; and heating the solder contact to abovethe second reflow temperature and below the first reflow temperature,such that the first component ensures a minimum spacing between the leadfinger and the die, and the second component ensures adhesion betweenthe solder contact and the lead finger, the method further comprisingproviding a solder repellent surface adjacent to a portion of the leadfinger that is in contact with the second component of the soldercontact.
 17. A method of fabricating a package for a semiconductordevice, the method comprising: providing a die having a pad and a soldercontact in contact with the pad, the solder contact comprising, a hardcore component having a first reflow temperature, and a coatingcomponent having a second reflow temperature lower than the first reflowtemperature; providing a lead frame comprising a lead finger; andheating the solder contact to above the second reflow temperature andbelow the first reflow temperature, such that reflow of the coatingcomponent ensures adhesion between the solder contact and the die andthe lead finger, and the hard core component ensures a minimum spacingbetween the lead finger and the die, the method further comprisingproviding a solder repellent surface adjacent to a portion of the leadfinger that is in contact with the second component of the soldercontact.
 18. The method of claim 17 wherein the hard core component isselected from plastic or metal.
 19. The method of claim 18 wherein thehard core component is selected from copper or stainless steel.